1. Field of the Invention
This invention pertains to a method of treating staphylococcal infection in mammals, including humans. The method involves the simultaneous administration of a lysostaphin or other agent which attacks the glycine-containing peptide cross-links of the cell wall peptidoglycan found in staphylococci and an antibiotic, the antibiotic properties of which are mediated by its ability to affect the cell wall of the target staphylococci. This combined administration is effective in treating the staphylococcal infection, and at the same time suppresses the formation of strains resistant to lysostaphin or other peptidoglycan active agent.
2. Background of the Prior Art
Lysostaphin is a bacteriocin secreted by a staphylococcal strain isolated and originally named Staphylococcus staphylolyticus (now S. simulans). The production of lysostaphin is described in U.S. Pat. No. 3,278,378. Lysostaphin is an endopeptidase which cleaves the polyglycine cross-links of the peptidoglycan found in the cell walls of staphylococci. U.S. Pat. Nos. 3,398,056 and 3,594,284 describe improvements to culture medium and inoculation techniques for the production of lysostaphin.
The gene for lysostaphin from S. simulans has been sequenced and cloned, U.S. Pat. No. 4,931,390. Lysostaphin for use as a laboratory reagent has been produced by fermentation of a non-pathogenic recombinant strain of B. sphaericus, from which it is readily purified. The cloning and sequencing of the lysostaphin gene permits the isolation of variant enzymes that have properties similar to or different from those of wild type lysostaphin. One such altered enzyme, bearing a single amino acid change, has been characterized and shown to have potent anti-staphylococcal activity both in vitro and in an animal infection model. U.S. patent application Ser. No. 09/120,030, filed Jul. 21, 1998 and incorporated herein by reference. Other lysostaphin analogues, including naturally occurring enzymes of this type have been established as potent agents capable of addressing difficult to treat bacterial diseases caused by staphylococcal infection. Other peptidases with related activity are known. Thus lasA protease and achromopeptidase, reported in Kessler, et al., J. Biol. Chem. 268:7503–08 (1993) and Li et al., J. Biochem. 122:772–778 (1997), respectively, have anti-staphylococcal activity based on their digestion of glycine-containing cross-links in the peptidoglycan cell wall component. These agents may be used in this invention in place of lysostaphin.
The development of lysostaphin as an effective antibiotic to treat staphylococcal infection has been plagued, however, by a problem that is universal for antibiotic administration—the increasing development of antibiotic-resistant strains of mutant staphylococci. Already, a wide variety of staphylococcal infections resistant to various antibiotics that were previously the treatment of choice, including methicillin (methicillin resistant S. aureus are referred to as MRSA) and vancomycin-resistant strains (referred to as VISA) have been identified. Resistance to a wide variety of other antibiotics, not exhibited by sensitive staphylococci, has been noted as well. MRSA, as well as strains resistant to other antibiotics, are discussed at length in Stranden, et al., J. Bacteriology 179(1):9–16 (1997). Further difficulties are encountered in that MRSA tend to accumulate a variety of other resistances as well. Multiresistant MRSA are typically treated with vancomycin, The Staphylococci In Human Diseases, 158–174 (Grossley, et al., editors 1997). Vancomycin itself may be toxic. Additionally, vancomycin resistance has recently been detected in staphylococci infections.
The problem posed by the continuing development of antibiotic-resistant infectious agents, such as staphylococci, is more than the difficulty involved in treating any individual patient. Popular press, as well as scientific journals, have noted the alarming increase in the generation of resistant strains, due in part to indiscriminate use or over-use of antibiotics. Each time an individual is treated with an antibiotic, whether needlessly or reasonably, the chance that a strain resistant to that particular treatment will arise is increased. Resistant strains of staphylococci have become endemic in many hospitals and pose a life-threatening danger to patients already debilitated by other ailments who become infected after admission to those hospitals.
Numerous articles have noted the development of resistance to either lysostaphin or β-lactams, such as methicillin, and the relationship there between. Thus, DeHart, et al., Applied Environmental Microbiology 61, 1475–1479 (1995) noted the development of mutant S. aureus recombinant cells that were resistant to lysostaphin, but susceptible to methicillin. Similar phenomenon are reported by Zygmunt, et al., Can. J. Microbio. 13,845–852 (1966), Polak. et al., Diagn. Microbiol. Infet. Dis. 17:265–270 (1993) and Dickson, et al., Yale J. Bio. Med. 41:62–67 (1968). Each of these references, as well as later reports such as Ehlert, J. Bacteriology, 179:7573–7576 (1997) note that staphylococci that develop resistance to lysostaphin, either spontaneously or through induced recombination, become susceptible to methicillin treatment, and vice-versa. In all of these references, the uniform suggestion is to follow a course of administration of lysostaphin, even a short one, with administration of methicillin.
U.S. Pat. No. 5,760,026, commonly assigned herewith, employs a specific method for treating mastitis, by intramammary infusion of lysostaphin. The patent reports, Table ID and elsewhere, that a synergistic result is predicted when combining lysostaphin and a β-lactam to treat mastitis, based on an in vitro assay. The bovine mastitis model is not predictive of in vivo administration of antibiotics, and the synergistic effects reported in U.S. Pat. No. 5,760,026 have not been substantiated in an environment or model that would be reflective of in vivo administration to a mammal such as a human.
Those of skill in the art will be aware that there are a wide variety of staphylococcal strains. Many are resistant to conventional antibiotics, unlike sensitive strains. S. aureus strains are recognized as highly virulent and the most common single cause of serious systemic infections. Coagulase-negative staphylococcal species, although generally less invasive than S. aureus, are now responsible for a significant incidence of infections; particulary among debilitated or immunocompromised patients. As an example of such infection, one may point to endocarditis consequent to heart valve replacement. This is but one of a variety of intractable staphylococcal infections which are increasing due to the widespread use of antibiotics.
Accordingly, it remains an object of those of ordinary skill in the art to develop a method whereby even resistant staphylococcal infections in mammals, including humans, may be effectively treated by the administration of antibiotics. Desirably, this method is developed so as suppress the formation of strains resistant to the antibiotics used.